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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

284

SILVER ALLOYS

precipitation of i gram of fine silver requires 100 c.c. of the standard salt
plus 3 c.c. of the weaker standard salt solution, to every 100 c.c. of the
standard salt it will be necessary to add a quantity of sodium chloride '
corresponding with that contained in 3 c.c. of the weaker standard, i.e.,
(0-000542 x 3) gram if pure salt has been used, or (0-000557 X 3) gram
with sea-salt. After this new quantity of salt has been added and dissolved,
the resulting solution is tested to ascertain if any small correction is still

necessary.

The temperature of the standard solution, corrected in this way, is
noted. Each time it is used, it should be well shaken to render it homo-
geneous.

Actual test. We will suppose that the preliminary test of an alloy of
silver and copper by Volhard's method gives the approximate fineness 834.
A weight, i-201 gram, is taken, this containing very slightly more than i
gram of silver. Two pieces, each of this weight, are placed in two of the
bottles and treated with 8-10 c.c. of nitric acid (D 1-2) with the precautions
mentioned above. When cold, 100 c.c. of the standard salt solution is
run into each bottle, which is then shaken, the further procedure being
as described for the standardisation of the salt solution. If, in addition
to the 100 c.c. of the standard salt solution, i c.c. of the weaker standard
is required, the amount of the silver in the 1-201 gram of the sample will
be i-ooi gram, the fineness of the alloy being 833-4 (1201 : 1001 :: 1000:

833-4).

To obtain greater accuracy, the weaker standard salt solution may be

added in portions of 0-2 c.c. A practised observer, however, by comparing
the intensity of the cloud given by the check with that given by the sample,
can estimate accurately by the eye o-i c.c. of the weaker standard salt
solution. In special works dealing with this subject, tables compiled by
Gay-Lussac are given which render unnecessary the calculations. If it
happens that the addition of i c.c. of the weaker standard salt produces
no cloud, it is best to repeat the test with a larger quantity of the sample.

Gay-Lussac's method, although expeditious, is undoubtedly the most exact
of all and is universally employed for the control of silver coinage. Naturally
the accuracy of the results depends on the accuracy of the weighing and par-
ticularly on the accuracy with which the standard solution is measured (i drop
= 0-5 milligram Ag). It is necessary also to allow for variations of temperature,
since at different temperatures the amount of soditim chloride contained in
100 c.c. of the standard solution varies appreciably (i higher or lower may
introduce an error of about 0-2 in the fineness). To eliminate this cause of
error, Gay-Lussac tried weighing the standard solution instead of measuring
it, but found this procedure so much less expeditious that he discarded it and
compiled a table giving corrections for temperature. In practice the simplest
means of avoiding such an error is to carry out, at the same time as the test
on the alloy, a control test with pure silver.

In connection with this method it is also to be borne in mind that silver
chloride, as pointed out by Mulder,1 is not absolutely insoluble, a minimal
quantity remaining in solution and undergoing precipitation only by excess of
the reagent. Thus, if the amount of sodium chloride exactly sufficient to

1 Die Silberprobiermethode.librium of the balance is attained.     The filed piece should